Ethylene and propylene, light olefin hydrocarbons with two or three atoms per molecule, respectively, are important chemicals for use in the production of other useful materials, such as polyethylene and polypropylene. Polyethylene and polypropylene are two of the most common plastics found in use today and have a wide variety of uses both as a material for fabrication and as a material for packaging. Other uses for ethylene and propylene include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Steam cracking or pyrolysis of hydrocarbons produces most of the ethylene and some propylene. One of the disadvantages of steam cracking is the low ratio of propylene to ethylene. Hydrocarbons used as feedstock for light olefin production include natural gas, petroleum liquids, and carbonaceous materials including coal, recycled plastics or any organic material.
An ethylene plant is a very complex combination of reaction and gas recovery systems. The feedstock is charged to a cracking zone in the presence of steam at effective thermal conditions to produce a pyrolysis reactor effluent gas mixture. The pyrolysis reactor effluent gas mixture is stabilized and separated into purified components through a sequence of cryogenic and conventional fractionation steps. A typical ethylene separation section of an ethylene plant containing both cryogenic and conventional fractionation steps to recover an ethylene product with a purity exceeding 99.5% ethylene is described in an article by V. Kaiser and M. Picciotti, entitled, “Better Ethylene Separation Unit.” The article appeared in HYDROCARBON PROCESSING MAGAZINE, November 1988, pages 57-61 and is hereby incorporated by reference.
Methods are known for increasing the conversion of portions of the products of the the ethylene production from a zeolitic cracking process to produce more propylene by a disproportionation or metathesis of olefins. Such processes are disclosed in U.S. Pat. No. 5,026,935 and U.S. Pat. No. 5,026,936 wherein a metathesis reaction step is employed in combination with a catalytic cracking step to produce more propylene by the metathesis of C2 and C4 olefins obtained from cracking. The catalytic cracking step employs a zeolitic catalyst to convert a hydrocarbon stream having 4 or more carbon atoms per molecule to produce olefins having fewer carbon atoms per molecule. The hydrocarbon feedstream to the zeolitic catalyst typically contains a mixture of 40 to 100 wt-% paraffins having 4 or more carbon atoms per molecule and 0 to 60 wt-% olefins having 4 or more carbon atoms per molecule. In U.S. Pat. No. 5,043,522, it is disclosed that the preferred catalyst for such a zeolitic cracking process is an acid zeolite, examples includes several of the ZSM-type zeolites or the borosilicates. Of the ZSM-type zeolites, ZSM-5 was preferred. It was disclosed that other zeolites containing materials which could be used in the cracking process to produce ethylene and propylene included zeolite A, zeolite X, zeolite Y, zeolite ZK-5, zeolite ZK-4, synthetic mordenite, dealuminized mordenite, as well as naturally occurring zeolites including chabazite, faujasite, mordenite, and the like. Zeolites which were ion-exchanged to replace alkali metal present in the zeolite were preferred. Preferred alkali exchange cations were hydrogen, ammonium, rare earth metals and mixtures thereof.
European Patent No. 109,059B1 discloses a process for the conversion of a feedstream containing olefins having 4 to 12 carbon atoms per molecule into propylene by contacting the feedstream with a ZSM-5 or a ZSM-11 zeolite having a silica to alumina atomic ratio less than or equal to 300 at a temperature from 400 to 600° C. The ZSM-5 or ZSM-11 zeolite is exchanged with a hydrogen or an ammonium cation. The reference also discloses that, although the conversion to propylene is enhanced by the recycle of any olefins with less than 4 carbon atoms per molecule, paraffins which do not react tend to build up in the recycle stream. The reference provides an additional oligomerization step wherein the olefins having 4 carbon atoms are oligomerized to facilitate the removal of paraffins such as butane and particularly isobutane which are difficult to separate from C4 olefins by conventional fractionation. In a related European Patent No. 109,060B1, a process is disclosed for the conversion of butenes to propylene. The process comprises contacting butenes with a zeolitic compound selected from the group consisting of silicalites, boralites, chromosilicates and those chromosilicates and those zeolites ZSM-5 and ZSM-11 in which the mole ratio of silica to alumina is greater than or equal to 350. The conversion is carried out at a temperature from 500° C. to 600° C. and at a space velocity of from 5 to 200 kg/hr of butenes per kg of pure zeolitic compound. The European Patent No. 109,060B1 discloses the use of silicalite-1 in an ion-exchanged, impregnated, or co-precipitated form with a modifying element selected from the group consisting of chromium, magnesium, calcium, strontium and barium.
U.S. Pat. No. 6,867,341 to Abrevaya et al. teaches naphtha cracking using a catalyst comprising a molecular sieve having 10-membered rings with channels of length 0.1 to 0.3 micrometers and having a silicon to aluminum atomic ratio of about 20 to about 200. In particular, examples are presented showing that a high Si/Al2 ratio Ferrierite catalyst is more effective for naphtha conversion and gives higher yields of the desired products ethylene and propylene than other zeolites examined. Preferred operating temperatures in the range 650 to 670 C are indicated, and operating pressures should be as low as can be economically achieved.
U.S. Pat. No. 6,288,298 to Rodriguez et al. teaches cracking of a naphtha stream that contains a mixture of paraffins and olefins (for example, a product stream from a steam naphtha cracker or a FCC process) using a high silicon content SAPO-11 catalyst with AEL structure. Preferred operating temperatures in the range 500° C. to 600° C. are indicated. The SAPO catalyst is shown by example to have higher activity and selectivity for propylene than conventional FCC catalyst additives such as ZSM-5. U.S. Pat. No. 6,300,537 and U.S. Pat. No. 6,521,563, both to Strohmaier et al. (and both assigned to ExxonMobil) show similar results using a different preparation of high silicon SAPO-11 designated ECR-42.
U.S. Pat. No. 6,258,257 to Swan et al. teaches a two stage process for producing C2 to C4 olefins from gas oil in which the gas oil is first contacted with an FCC catalyst to produce an olefinic naphtha stream and this naphtha stream is then contacted with ZSM-5 or other small or medium pore zeolites at a temperature in the range 630° C. to 650° C.
U.S. Pat. No. 6,791,002 to Abrevaya et al. teaches use of a plurality of riser reactors attached to a common regenerator, allowing each riser reactor to contact an oil stream at different conditions of temperature and residence time. Unconverted intermediate products from catalytic cracking of naphtha are recycled to different riser reactors where they are contacted with catalyst under the appropriate reaction conditions.
All of the above prior art schemes suffer from the disadvantage that multiple reaction reaction steps are needed to effectively convert the feed into the desired products. This increases the complexity and cost of the reaction system, as well as increasing the amount of material that must be collected in the separation system for recycle to the reactors. The overall effect is to increase the capital and operating costs of the catalytic naphtha cracking process.
It is difficult in naphtha cracking to obtain high selectivity to ethylene and propylene, while maintaining high conversion. Improvements in catalysts and processes that accomplish this are therefore desirable.